The present invention relates to an automatic gain control (AGC) circuit and, more particularly, to an AGC circuit which shortens the pull-in time upon switching an input signal or turning on the power supply.
Generally in an AGC circuit, as the loop band ldecreases, the pull-in time becomes longer upon switching an input signal or turning on the power supply. For this reason, the pull-in time is desired to be shortened.
FIG. 3 shows an AGC circuit using a conventional method of shortening the pull-in time upon switching an input signal, which is disclosed in Japanese Patent Laid-Open No. 5-83053. In the AGC circuit shown in FIG. 3, a peak hold circuit 32 detects the peak voltage of an output from an amplifier 31 to output a peak hold voltage Vp. An operational amplifier 33 arithmetically processes and amplifies the peak hold voltage Vp from the peak hold circuit 32 and a reference voltage Vr to output a gain control voltage Vc. An analog-to-digital (A/D) converter 35 converts the gain control voltage Vc into a digital signal and stores the digital signal in a memory 36 in accordance with a write enable signal WE and an address signal ADR.
The digital signal stored in the memory 36 is converted into an analog voltage (gain control voltage Vm) by a digital-to-analog (D/A) converter 37. A system switching circuit 38 selects one of an output from the D/A converter 37 and an output from the operational amplifier 33 in accordance with an external selection signal SEL, and outputs the selected one to a gain switching circuit 34. The gain switching circuit 34 controls the amplification factor of the amplifier 31 on the basis of an output from the system switching circuit 38.
In the AGC circuit having this arrangement, a digital signal for the gain control voltage Vc corresponding to an input signal is stored in the memory 36 in accordance with addresses corresponding to input signals having various fixed amplitudes. When the input signal is switched, the system switching circuit 38 is switched, and then the amplification factor of the amplifier 31 is controlled on the basis of the digital signal for the gain control voltage Vc corresponding to the input signal that is read out from the memory 36.
The conventional AGC circuit uses the A/D converter 35, the memory 36, and the D/A converter 37 to increase the number of components, which does not allow cost reduction. In addition, noise upon switching the gain switching circuit 34 may be applied to the amplifier 31. Upon turning on the power supply, the above AGC circuit requires a long pull-in time, like a general AGC circuit.